The area ratio of the land and sea of Japan is one to eleven, while the area ratio of the land and sea of the whole world is three to seven, and thus, a role played by the ocean development is important for the growth of Japan in the 21st century. In the field of the ocean development, supervising and monitoring techniques for the development of seabed resources, the research of marine environments and organisms, and the monitoring of earthquakes, the facility management for power generation on the sea and a marine farm that are expected to be increasingly built in the near future, and the like are highly expected. In order to efficiently and safely practice the supervising and monitoring, communication technology in seawater plays an important role. Currently, in most of cases, communication with a home base station on the sea or land is made through wired cables, but this configuration imposes a large limitation on devices installed on seabed, the action ranges of underwater robots, and the like. Thus, for communication with installation points of the devices on seabed and communication with the underwater robots, the research and development of underwater acoustic communication techniques that enable realization of wireless communication have been actively conducted.
Such underwater acoustic communication has been widely and generally used because it is capable of achieving both of a broad communication region extending to a distance longer than several km and a transmission speed sufficient for the transmission of control commands and the transmission of images (namely, a transmission speed>several kbps).
For example, an underwater acoustic communication technique is disclosed in Patent Literature (PTL) 1. In an underwater communication method disclosed in PTL 1, a wave receiver convers an acoustic signal having been propagated in water into an electrical signal, and a receiving circuit amplifies a weak acoustic electrical signal from the wave receiver. The receiving circuit frequency-analyzes FSK (Frequency Shift Keying) modulated waves of a received signal composed of a start pulse, data pulses, and an end pulse to thereby detect the presence or absence of the start pulse. Upon confirmation of the presence of the start pulse, the receiving circuit makes a code determination using the result of the frequency analysis by which detected signals have been temporally separated.